The problem asks to identify and count the total number of deactivating groups for aromatic electrophilic substitution from a given list of functional groups.
Concept Used:
The reactivity of a substituted benzene ring towards an electrophile is determined by the nature of the substituent group. Groups are classified as either activating or deactivating based on their ability to donate or withdraw electron density from the ring.
Activating Groups: These groups donate electron density to the benzene ring, making it more nucleophilic and thus more reactive towards electrophiles than benzene itself. They are typically ortho, para-directing. This electron donation occurs primarily through the resonance effect (+R or +M) or the inductive effect (+I). Examples include \(-\text{OH}\), \(-\text{OR}\), \(-\text{NH}_2\), \(-\text{NHR}\), and alkyl groups (\(-\text{R}\)).
Deactivating Groups: These groups withdraw electron density from the benzene ring, making it less nucleophilic and less reactive towards electrophiles. They are generally meta-directing (except for halogens). This electron withdrawal occurs through the resonance effect (-R or -M) or the inductive effect (-I). Examples include \(-\text{NO}_2\), \(-\text{CN}\), \(-\text{SO}_3\text{H}\), and carbonyl groups (\(-\text{CHO}\), \(-\text{COR}\)).
Step-by-Step Solution:
Step 1: List the unique functional groups shown in the image.
Step 2: Analyze each group to determine if it is activating or deactivating.
1. Acetyl group (\(-\text{C}(=\text{O})\text{CH}_3\)):
The carbonyl group is strongly electron-withdrawing. It withdraws electron density from the ring through both the inductive effect (-I) due to the electronegative oxygen atom and the resonance effect (-R). Therefore, it is a deactivating group.
2. Methoxy group (\(-\text{OCH}_3\)):
The oxygen atom is electronegative and exerts an electron-withdrawing inductive effect (-I). However, it has lone pairs of electrons that can be delocalized into the benzene ring via the resonance effect (+R). The +R effect is stronger than the -I effect, leading to a net donation of electron density to the ring. Thus, it is an activating group.
3. Acetamido group (\(-\text{NHC}(=\text{O})\text{CH}_3\)):
Similar to the methoxy group, the nitrogen atom has a lone pair that it can donate to the benzene ring (+R effect). Although this lone pair is also in resonance with the adjacent carbonyl group (which reduces its availability for the ring), the overall effect on the benzene ring is electron-donating. Therefore, it is an activating group (though less activating than \(-\text{NH}_2\)).
4. Methylamino group (\(-\text{NHCH}_3\)):
The nitrogen atom has a lone pair that strongly donates electron density to the ring via the resonance effect (+R). This makes it a strong activating group.
5. Cyano group (\(-\text{C}\equiv\text{N}\)):
This group is strongly electron-withdrawing due to the high electronegativity of nitrogen and the sp-hybridized carbon. It exhibits both a strong inductive effect (-I) and a resonance effect (-R), pulling electron density away from the ring. Therefore, it is a deactivating group.
Step 3: Count the number of deactivating groups.
Based on the analysis in Step 2:
Activating groups are: \(-\text{OCH}_3\), \(-\text{NHC}(=\text{O})\text{CH}_3\), and \(-\text{NHCH}_3\).
Deactivating groups are: \(-\text{C}(=\text{O})\text{CH}_3\) and \(-\text{C}\equiv\text{N}\).
There are two deactivating groups in the given list.
Final Result:
The total number of deactivating groups among the given options is 2.
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